The field of this invention is apparatus for controlling flow under throttling conditions in order to regulate the level, temperature or pressure in process control applications.
Conventional valve throttling trim for such purpose usually consists of a lathe-turned plug of generally parabolic shape axially displaced in, or axially shiftable to close against or open from, a cylindrical orifice. The annulus formed between the outer periphery of the plug and the inside diameter of the orifice provides the desired flow area at a given lift position. The relation between such a flow area at any given lift to the flow area at maximum lift determines the flow characteristic of such a trim.
The combination of effective flow area and velocity headloss defines the flow capacity, usually expressed in Cv, where 1 Cv is the flow of 1 US gpm of water passing through a restriction under a pressure drop of 1 psi. As derived from the Darcey equation, wherein C is a contraction coefficient and K is the velocity head-loss coefficient: ##EQU1##
In conventional trim systems, since the velocity head-loss coefficient, or fluid resistance, is for all practical purposes constant (usually K .apprxeq. 0.7), the flow characteristic can only be determined through variations in flow area, which of course requires precision machining of the valve plug. Furthermore, the relatively smooth flow path between a parabolic plug and an orifice can lead to pressure recovery and therefore cavitation with liquids. In addition, most process control systems, due to pump droop and line resistance in series with the valve, require that the pressure drop across the valve should rise in inverse proportion to the square of the decrease in flow rate (i.e., at 25% flow the pressure drop can be 16 times higher than at 100% flow), and that the valve should therefore have a flow characteristic commonly called "equal percentage" to compensate for the non-linearity of pressure drop. But the achieving with accuracy of such characteristic, i.e., of the wanted variations in flow area, again requires the precision machining of a complex curvature on the valve plug. Further, since ##EQU2## and since at low flow rates the differential pressure (.DELTA.p) is high, the conventional trims, with their relatively low velocity head-loss coefficient K, produce excessive fluid velocities at said low flow rates. As above indicated, these excessively high velocities cause cavitation or erosion of the trim with liquids. Similarly, since sound pressure level (SPL) = f(V).sup.8, with the conventional trims the high velocities occurring at the low flow rates cause substantial aerodynamic noise with gases.
In other apparatus such as of prior U.S. Pat. Nos. to Self 3,514,074 and 3,513,864, and to Cummins 3,529,628, the trims do provide a high resistance or velocity head-loss coefficient K, but through the use of labyrinthe flow passages, or passages having repeated, velocity-reducing changes of direction, that are formed between stacked washers such that the flow must be radially of the stack, or in a direction essentially perpendicular to the plug axis. Thus, and since the Self and Cummins washer stacks are uniformly constructed end to end, they have a linear flow characteristic, or linear increase in flow area with valve opening, that results, similarly as with the conventional trims, in substantially higher flow velocities at low flow rates.
Furthermore, all of the pressure drop across the labyrinthe washer stack flow passages occurs at the same time across the leading tip of the solid cylindrical valve plug employed therewith, this since the next stack opening -- i.e., the one not yet exposed by the plug lifting -- is at the low downstream pressure level, whereby substantial wear is experienced at the beforementioned plug tip point.
Still another disadvantage of the last mentioned prior art devices is that each set of flow paths constituted by a pair of perforated plates, such as of Self. U.S. Pat. No. 3,513,864, is required to be separated in the vertical direction from the next set by a solid wall, which produces a discrete pause in the rate of flow increase, which pause is detrimental for automatic control purposes.
Yet another shortcoming of the Self-Cummins devices is their requirement of relatively large pressure vessels to house the cylindrical disc stack. For example, for a 2 inch plug diameter having a maximum Cv of 30, the Self-Cummins stacked disc valves require a valve bonnet opening or flange of about 4 inches, or of about twice the size, and three times the cost and weight, of the conventional parabolic trim.
This invention solves the foregoing problems by the provision novelly of a valve trim whose specific fluid resistance changes in direct but inverse proportion to valve stroke. That is, as the conductive flow area of the valve trim gradually increases, in going from the closed to the open valve position, the trim impedance, or the total amount of resistance to fluid flow, gradually decreases from a maximum level near the closed valve position to a minimum value at the full open position, or the valve stroke position corresponding to the maximum obtainable flow passage area.
Among the many advantages of the invention trim, then, are:
1. The resistance of the valve trim is high when the valve is near-closed and fluid flow through the valve is low, i.e., when the typical pressure of the fluid is high and the resultant pressure drop across the valve is near its maximum level. The high resistance of the trim in the near-closed stroke position accomplishes the required kinetic energy conversion without incurring the excessive erosion or cavitation experienced with the conventional valve trims.
2. The resistance of the trim is low when the valve is near full-open, at which stroke position the fluid pressure at the valve inlet is greatly reduced (due to droop in pump characteristic, pipeline resistance, etc.), little pressure drop is required, and the actual physical behaviour of most controlled fluid systems is very nicely complimented.
3. The inverse relationship between flow area and resistance coefficient gives the invention valve trim an exceptional rangeability, or high ratio between maximum and minimum controllable flow. A typical valve trim of the invention could have 20 fluid conducting passages to be opened up consecutively while at the same time decreasing the resistance from a velocity head-loss coefficient k of 20, when only one channel is exposed to flow, to .apprxeq.1, when all channels are opened up. The amount of fluid "Q" passing a valve being expressed by the Darcey equation as ##EQU3## The ratio of Q maximum to Q minimum of the invention device accordingly is ##EQU4## or nearly 90:1 for the stated area ratio of only 20:1. This contrasts with a rangeability of only 20:1 for a conventional valve trim having the same flow area ratio.
4. The valve trim hereof is enabled by the invention to be manufactured or fabricated without the precision machining required for, and hence without the tolerances handicapping, conventional trim, and so that all flow passages and their respective specific fluid resistances are uniform and reproducible. Not only are desired flow characteristics thus assured or obtained with a high degree of accuracy, but also the invention user is enabled to predict mathematically the exact flow relationship to be obtained with a selected pattern of the flow passages.
5. In the configuration for liquid media, the invention device's flow characteristic is a function of both a variable flow area and an inverse variable velocity head-loss coefficient K. The invention valve plug, then, not only does not require a precision contour but also provides for an increase in internal fluid resistance with decrease in flow -- i.e., increase in pressure drop across the valve -- thus assuring a nearly constant throttling velocity, with absence of erosion and noise.
6. The invention flow passages comprise a plurality of flow paths parallel to the plug axis and in which a number of restrictions are provided in series to achieve a high K factor. The series-restricted flow paths are formed in one disclosed embodiment as slots in the engaged surface or face of one and closed by the smooth bore or periphery of the other of the interfitting or slidingly engaged plug and seat ring, whereby both the number of the active flow paths and their specific resistances vary with the lift or stroke of the valve.
7. Whereas with the aforementioned Self and Cummins trims the substantial plug wear inducing "leakage flow" passes directly and unrestrictedly through the valve plug -- seat ring gap or opening, any such flow with the invention apparatus is confined in a prolonged, narrow or shallow gap or channel or channels formed between a cylindrical plug and seat ring and in which the leakage flow is continually disrupted and interrupted by successive or series passageway restrictions, slots or grooves.
8. A surprising, extraordinary economic benefit, or great cost and space saving, is realized relative to the mentioned prior art trims; these require relatively large pressure vessels to house the cylindrical discs that are required, whereas with this invention the trim size is enabled to be nearly identical to that of conventional parabolic trim. For example, as embodied in a 2 inch diameter plug device the invention trim has a maximum c.v. of 30. A valve bonnet having an opening of about 4 inches diameter is required to accommodate a Self or Cummins patent stacked disc type valve of identical capacity. The weight of a 4 inch opening or flange bonnet is about three times the weight of the 2 inch bonnet, and the directly related cost reduction by the invention is similarly to about one third the cost of the valve and housing assemblies of the mentioned patents.
9. In case a high pressure drop independent of flow rate is required, such as for boiler feed-water recirculating valves, the invention can accommodate a fixed restriction in series with the movable valve trim. Such fixed restriction, absorbing more than 75% of the pressure drop at maximum flow, shifts the burden away from the movable valve plug, and so results in substantial additional cost benefit or savings in regard to the power required of the valve operator.
10. The invention enables a wider rangeability between minimum and maximum controllable flow, due to the combined variation of both K factor and flow area, than any conventional or prior art trim, this allowing substantial energy savings in liquid pumping systems, since very low valve pressure drop can be tolerated at the wide open valve position without distortion in the overall control valve gain.
11. The individual flow paths of the invention trim may be interconnected, in order to provide for a rapidly increasing flow area with successive increases in the flow path restrictions, in order to accommodate increase in volume following a reduction in pressure of gases, or to reduce velocity with liquids. The invention trim thus achieves or assures low outlet velocities, and resultant low aerodynamic noise levels, and thereby effectively combats, or solves, noise pollution problems.
12. As contrasted with the cast or milled solid metal trim of U.S. Pat. No. 3,908,698, requiring fluid passages entirely accessible from the circular interface between plug and seat ring and thereby a more costly manufacture and a less than full utilization of available flow capacity, the improved trim hereof overcomes such deficiencies through the use of stamped plates which, when stacked in certain or predetermined relationship, provide the exact desired characteristic in area increase and in resistance decrease as a function of valve stroke. Indeed, accuracy and repeatability are guaranteed by the present trim improvement, so long as the same stamping die is utilized.
13. With the perforated, stamped plate trim design of this invention, a larger or higher utilization of cross-sectional trim surface for flow area is achieved than heretofore possible or practical, by providing or stamping out additional flow passages within supporting segmental sections of the plug or seat ring.